Background prior art is described in Protein Science, Vol 15, 2006, J A Marsh et al, “Sensitivity of secondary structure propensities to sequence differences between alpha- and gamma-synuclein: Implicationd for fibrillation”, 2795-2804; and in silico Biology, Vol 7, 2007, S Inicula-Thomas et al, “Correlation between the structural stability and aggregation propensity of proteins”, 225-237. We have previously described, in WO 2004/066168 and in WO 2005/045442, techniques for predicting the rate of aggregation/solubility of proteins in their native, unfolded state. These techniques are useful, for example, in predicting aggregation-resistant mutational variants of unstructured polypeptide chains but they are not in general applicable to the prediction of aggregation in structured (folded) proteins. However, the aggregation of proteins from their folded state is important for many diseases, and the accurate prediction of this phenomenon is considered a difficult problem which, heretofore, has not been solved. We will describe a tool to address this problem; there are many applications of the tool, including rational design of drugs as well as protein production techniques.